In a known manner, as illustrated in FIG. 1, a propulsion system of an aircraft, for example connected under the wing by means of a mast, comprises a nacelle 14 in which a power plant that drives a fan 16 is arranged in an essentially concentric manner. The longitudinal axis of the nacelle is referenced 18.
The nacelle 14 comprises an inside wall that delimits a pipe with an air intake at the front.
To limit the impact of noise pollution close to airports, techniques have been developed to reduce the noise emitted by an aircraft, and in particular the noise that is emitted by a propulsion system, by arranging, at the walls of the pipes, panels, coverings or structures whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators. In a known manner, a panel for the acoustic treatment comprises—from the outside to the inside—an acoustically resistive porous layer, at least one alveolar structure, and a reflective or impermeable layer.
For the moment, because of various constraints, for example shaping or compatibility with other equipment, the coverings are provided in particular at the inside wall of the nacelle in a limited zone that is distant from the air intake and the air discharge.
To increase the effectiveness of the acoustic treatment, one solution consists in expanding the surfaces that are covered by the acoustic covering and in extending it at the level of the air intake. However, at the air intake or the lip of the nacelle, the acoustic treatment should not affect the operation of systems that make it possible to prevent the formation and/or the accumulation of ice and/or frost that are necessary in these zones.
These systems are divided into two families, the first called defrosting systems that make it possible to limit the formation of ice and/or frost, and the second called de-icing systems that limit the accumulation of ice and/or frost and that act once the ice and/or frost is formed. Hereinafter, a frost treatment system is defined as a defrosting system or a de-icing system, the term frost encompassing frost or ice.
This invention relates more particularly to a frost treatment process that consists in using the hot air that is taken from the engine and fed back at the inside wall of the leading edges.
According to an embodiment that is known and illustrated in FIG. 2, a nacelle 14 comprises, on the inside, a partition that is called a front frame 24 that with the air intake 22 delimits a pipe 26 that extends over the entire circumference of the nacelle and that has an essentially D-shaped cross-section.
This pipe 26 is supplied with hot air by a system of nozzles or a feed pipe 28 that is located at a point. This hot air makes a 360° passage around the leading edge, and besides a centrifugal effect, the hot air circulates on the outer side of the leading edge as illustrated in FIG. 3B, on which the de-icing capacity was shown as a function of s, with s=0 corresponding to the top part of the air intake as illustrated in FIG. 2, the value of s being positive and increasing on the outer side of the nacelle based on distance from point 0 and the value of s being negative and increasing by absolute value on the inner side of the nacelle based on distance from point 0.
If hot air is injected at a point that is located at 180° (0 corresponding to the highest point of the nacelle), a de-icing capacity is obtained that is not homogeneous over the circumference that quickly expands to reach a maximum value at 220°, and then a gradual reduction over the remainder of the circumference, as illustrated in FIG. 3B. Thus, a discontinuity of frost treatment at the lowest level is noted.
However, as illustrated in FIGS. 4A and 4B, the zone that requires the most significant frost treatment is located at the inside edge of the air intake over the entire circumference to limit the risk for the power plant of ingesting ice particles.
In the case of an acoustic treatment at the air intake, as illustrated in FIG. 2, an acoustic treatment panel 30 is to be placed at the level of the inner side of the nacelle that is also the zone that should be treated most effectively relative to the frost.
However, the acoustic treatment panel 30 that consists of air-containing cells acts as a thermal insulator that limits the effect of the frost treatment. One solution then consists in increasing the temperature of the air of the frost treatment so as to effectively treat the air intake. However, to withstand significant temperatures, it is advisable to use materials whose mass is higher than that of composite materials; this tends to increase the on-board mass and therefore the energy consumption of the aircraft.
So as to attempt to make acoustic and frost treatments compatible, one solution described in the documents EP-1,103,462 and U.S. Pat. No. 5,841,079 provides holes in the reflective wall so that the hot air penetrates into the cells of the acoustic covering.
However, this solution is not satisfactory for the following reasons:
The cells of the alveolar structure that comprise one or more holes at the reflective layer are less capable in terms of acoustic treatment, with the waves dissipating less well in said cells. To reduce this alteration, one solution consists in reducing the cross-sections of holes. In this case, the air volume at a constant flow rate is reduced, making the de-icing less effective. Furthermore, these holes with reduced cross-sections can be plugged more easily, which eliminates the de-icing function in the corresponding zone.
The document EP-1,232,945 describes an acoustic treatment that comprises an acoustically resistive porous layer, a reflective layer, and, between the two, an alveolar structure that comprises a number of clusters of cells. Thus, according to this document, the acoustic treatment is performed at cell clusters, and the frost treatment enters the cell clusters.
According to one embodiment, the clusters come in the form of strips of cells that are parallel to one another and perpendicular to the longitudinal axis 18 of the nacelle, whereby each strip is delimited by two lateral partitions. With the strips being spaced apart, a passage that is bordered by the side walls of the strips is obtained between two adjacent strips. According to a first variant, a reflective layer that is common to all of the strips and scoops for introducing air into the passages is provided. According to another variant, each strip comprises a reflective layer, a bent part being provided to cover several strips.
Even if it makes it possible to make an acoustic treatment co-exist with a frost treatment, this solution does not make it possible to optimize the frost treatment in the most sensitive zones.
According to another significant constraint, the alveolar structures should be relatively airtight between two points that are spaced apart in the longitudinal direction so as not to create an air flow between these two points inside the acoustic treatment panel that can generate a perturbed stream at the aerodynamic surface.